1. Field of the Invention
The present application relates to a surface mountable thermistor, i.e., SMD type thermistor, comprising electrically conductive polymer, such as a positive temperature coefficient (PTC) device or a negative temperature coefficient (NTC) device, to provide over-current protection. In addition, the surface mountable thermistor is capable of detecting abnormal ambient temperatures.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 And 37 CFR 1.98
Because the resistance of conductive composite materials having a PTC characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperatures, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g. at least 102Ω), so as to suppress over-current and protect the cell or the circuit device.
In high density circuit design and manufacturing, it is demanded that the protection devices have to be light, thin and small, and have to be surface mountable for being secured to circuit boards. Therefore, the thermistors having organic polymer have been designed to be various surface mountable electronic devices. However, when the device is applied to high temperature environment such as LED applications, the hold current thereof cannot increase due to the limitation of device dimensions and inferior heat transfer. Moreover, if the device is characterized in high thermal insulation, then the problem that the device is not sensitive to ambient temperature will occur.